I've been trying to get my 12sb repaired. It runs excellent but the trans would not shift into 5th. 4th was difficult also. I tried adjusting the cable and that allowed me to shift 2-5 with 5th being nearly impossible.

I removed the trans and disassembled it. I discovered some some moisture had gotten into the grease. There is no rust or obvious damage.

There are a few reasons the shifting is too stiff

1) the shift collar doesn't slide freely on the holder. The holder is plastic, do these swell?

2) The shift keys pass through a washer separating each gear, I suppose this gives positive engagement. 1-3 has a nice easy click 4-5 are stiff. I tried polishing the shift key but it didn't seem to help. Any ideas?